Process for the production of monobasic aluminum sulphite



Patented Aug. 28, 1934 UNITED stars r vr DFFE I l PROCESS FOR THE PRODUCTION OF MONO BASIC ALUMHNUM SULPHITE Erich Wiedbrauck we as Biiche, Essen-Ruhr. Germany, assignors to firm Th. Geldschmidt A.-G., Essen-Ruhr, Germany N0 Drawing. Application January 23,

21 Claims. (Cl. 23-129) This. invention relates to a process for the pro- ,duction of monobasic aluminum sulphite.

The technical solutions-usually containing free sulphur dioxide-of aluminum sulphites obtained by. the action of aqueous sulphurous acid on aluminiferous minerals, aluminum hydroxide or other aluminiferous compounds, or by converting technical grades of aluminum salts with the aid of sulphites, or by any other means, are always contaminated by extraneous constituents. Consider'able difficulty is experienced in preparing from such solutions pure aluminum compounds which meet all requirements with regard to quality, especially freedom from silica and iron.

Attempts have already been made to produce basic aluminum compounds, free from impurities, from technical aluminum sulphite liquors, by simple hydrolysis, that is, by removing the sulphurous acid present in the, solution. So far as their iron content is concerned, the aluminum compounds obtained in this extremely simple way are often satisfactory, provided certain precautions, such as exclusion of air, are observed. Nevertheless, they always contain considerable amounts of silica, according to the nature of the original solution and ranging up to 10 per cent (referred to A1203) which preclude their employment for most purposes, and especially for the manufacture of metallic aluminum. 7 Attempts have also been made to separate the silica from the alumina in a suitable manner. For instance, fractional precipitation, or fractional hydrolysis, and the method of repeatedly dissolving and reprecipitating'the basic aluminum sul hite first obtained, have been tried. In all the e operations the hydrolysiswas brought about by removing the sulphurous acid from the liquors, for example by boiling or evacuation.

The basic aluminum sulphites thereby obtained are, for the most part, compounds without fixed crystalline structure and composition. Depending onthe conditions of precipitation, they contain larger or smaller amounts of free sulphurous acid, and S02 and A1203 occur in these compounds rarely in simple molecular proportions. For instance, it isvery difficult to obtain the only known dibasic salt: A1203.S02.4H20, which was first preparedby Goggingsberg. On page 61 of his book Die D'arstelluug von Bisulphiten und Sulphiten Schlitz states that only a single salt, namely the dibasic aluminum sulphite, exists.

Applicants have found, surprisingly, that there is also a monobasic aluminum sulphite, which takes the form of a solid, crystallinecompound ,Which always contains, in addition to water, 2

molecules of S02 to 1 molecule of alumina. This substance was hitherto entirely unknown.

The monobasic aluminum sulphite is formed when aluminum sulphite solutions are heated under conditions under which they completely or partially retain their S02 content, the presence of at least 2 molecules of S02 to 1 molecule ofAlzOa being essential. A pure crystalline monobasic aluminum sulphite, separates out from the solu-- tions, the yield being almost quantitative.

Retaining the S02 content of the aluminum sulphite solution is achieved in the simplest manner, for example by heating the solution in a closed vessel under pressure. The operation may also be conducted Without the application of pressure, in which event, the ratio of alumina to sulphurous acid, within a definite temperature range, must always be such that an adequate amount of sulphurous acid that is, at least 2 molecules of $02 to 1 molecule of AlzO3is present throughout the formation of the monobasic aluminum sulphite.

If an aluminum sulphite solution be heated in a closed vessel, and therefore under S02 pressure, the prescribed condition for the formation of monobasic aluminum sulphite is directly fulfilled, since no sulphurcus acid can escape.

The operation can also be conducted by first expelling a portion of the free S02, usually present in the aluminum sulphite solutions, by suitable means such as warming the solution, evacuation and so forth, and then heating these solutions, which have been impoverished in S02, in closed vessels under pressure. The deposition of the monobasic aluminum sulphite solutions begins at temperatures above 0., and proceeds more rapidly as the temperature rises. For example, it is complete in i5 minutes at 100 0., about per cent of the alumina originally dissolved as sulphite being deposited.

The conditions are d-iiferent when the heating is conducted in open vessels, that is, without pressure, since in such case the sulphurous acid can escape. Althoughsulphites are likewise formed, unlessfspecial precautions be taken, these sul phites are highly basic, as a rule, owing to the comparatively small amounts of sulphu-rous acid available In such case, the temperature at which the reaction. occurs can be maintained at. sucha level that suficient sulphurous acid for the formation of the mcnobasic sulphite is availablethroughout the entire reaction period. This condition is fulfilled by operating at deposition tem-- peratures not exceeding 90 C. The influence of the deposition temperature on the composition of the resulting deposition product is shown by the following experimental figures.

Composition of the reaction product- Deposition temperatures 0.

11101 A120 inol S01 The monobasic aluminum sulphite obtained by deposition under pressure, or without pressure, is a very pure compact, crystalline salt. Its composition, in the moist state after suction filtration, is usually as folows:

Per cent the remainder being water.

In drying, the salt loses about 19 to 21 per cent of water, the dry salt containing, for example:

Per cent A1203 39.72 S02 38.l3 remainder water.

The new salt is comparatively stable, sparingly soluble in water, soluble in aqueous solutions of S02, and soluble in acids, S02 being liberated. It reacts with alkaline substances. On heating, S02 is given olT.

Example 1 A solution containing 51.1 grams of A1203 and 192.6 grms. of S02 per litre is heated to 70 C., the excess of S02 beyond 117.3 grms. per litre in the solution being driven off. The solution treated in this manner is heated for 3 hours in a closed, lead-lined autoclave, the pressure rising to 3-4 atmospheres. After cooling, the deposited monobasic aluminum sulphite is filtered on, leaving the solution still containing 4.5 grms. of A1203 per litre. Consequently, 91.2 per cent of the alumina originally present is recovered as monobasic aluminum sulphite.

Example 2 A solution containing 33 grins. of A1203 and 134 grms. of S02 per litre is heated at C. for 45 minutes in a closed, lead-lined autoclave agitator, the pressure rising to 7 atmospheres. The reaction mixture is treated in the same manner as in Example 1. The filtrate still contains 1.4 grms. of A1203 per litre, corresponding to a deposition of more than 95 per cent.

Example 3 minum compound can be obtained from impure aluminum sulphite liquors by treating them in accordance with the present process for the production of monobasic aluminum sulphite, the monobasic sulphite of well-defined composition in respect to S02 and A1203 being deposited. This aluminum sulphite is practically free from Si02 and is also very low in iron. In order to reduce this low iron content still further, the deposited monobasic aluminum sulphite can be redissolved in aqueous sulphurous acid, and then redeposited from the solution by heating under conditions enabling the S02 content to be completely or partially retained.

In order to eliminate the silica and iron from the aluminum sulphite liquors in a practically complete manner, by simply expelling the S02, the above described mixture-40w in SO2-of aluminum compounds can first be produced in the usual manner. This mixture is then dissolved in aqueous sulphurous acid, whereupon the monobasic aluminum sulphite is deposited from the solution by heating under conditions completely or partially maintaining the S02 content. The deposited compact, granular salt, after washing with water, is of a high degree of purity. The residual, sulphurous mother liquors contain all the impurities, especially the iron and silica, and they may be employed, for example, for decomposing further quantities ofaluminous compounds. In order to minimize heat consumption in this recrystallization, it is advisable to employ concentrated aqueous sulphurous acid in dissolving the deposited basic aluminum sulphite mixture containing a small amount of S02. In this manner, aluminum sul-. phite solutions with up to 170 grms. of A1203 per litre can be obtained.

One advantage of the deposition process of the present invention consists in that the consumption of heat is considerably smaller than in the method of deposition by boiling off the S02, which is accompanied by the evaporation of a not inconsiderable amount of water. The

almost quantitative deposition of the monobasic aluminum sulphite requires only a short heating of the liquor, at 80 C. for example, in a closed autoclave, without any evaporation of liquid being needed. This point is important in relation to the treatment of the dilute aluminum sulphite liquors usually obtained industrially.

It has also been observed that the density of the deposited monobasic aluminum sulphite increases with rising deposition temperature. This is particularly noticeable when the resulting moncbasic aluminum sulphite is calcined for the production of alumina, as is shown by the following experimental examples:

Specific gravity of alumina-in the loose stateprepared by calcining:

(1) Basic aluminum sulphite mixture, low

in S02 0.48 (2) Monobasic aluminum sulphite, deposited at 80 C 0.77 (3) Monobasic aluminum sulphite, deposited at 115 C 1.14

Apart from the deposition temperature, the

grain size of the deposited monobasic aluminum sulphite is influenced by such factors as agitation, rate and duration of stirring, concentration of the solution in respect to A1203 and S02, etc. Hence, by selecting the conditions of deposition a finer or coarser monobasic aluminum v memes sulphite and therefore alumina well 'can :be obtained at will. 7

Example '4 A solution obtained bydecomposing clay j autoclave.

filtered off and washed, and now contains only aqueous sulphurous acid, .gave the following compositiona Per-cent A1203 3131 "Feib'g 0.30 s02 15.40 S03 Q -Q 0. 21 and S102 03020 all by Volume.

Referred to 100-parts of ammin tne impurities have therefore decreasedfrom:- I

10.0 parts of F6293 in the solution to 0.44 in i the salt 0.61 parts of =Si02 the salt,

Approximately 95 per cent of the alumina originally present has been deposited, as monobasic aluminum 'siil'pliite, from the liquor.

The resulting monobasic aluminum sulphite, which still contains 0.11 per cent of R203, is redissolved in aqueous sulphurous acid, and the solution is reheated to 100 0., in the pressure After cooling, the deposited salt is the solution to 0.00 in 0.008 per cent of iron.

Example 5 The following basic aluminum sulphite is ob- A tained by boiling an aluminum sulphite liquor:

Per cent A1203 14.39 F8203 0.04 I Si02 0.14 S02 5.18 S03 1.06

Of this damp salt, 250 grins. are dissolved at C. with 800 grms. of water and 200 grms. of S02, the excess of the latter being expelled by heating up to C. The solution is then heated at 100 0. (pressure 5-6 atmospheres) for 1 hour in an acidproof autoclave. The filtered and washed monobasic aluminum sulphite has the following composition while in the moist state in which it leaves the suction filter:

Per cent A1203 25.44: 502 30.50 S02 0.83 F8203 0.0021 'Si02 0.01

' Referred to 100 parts of alumina, the impurities have therefore decreased by the redeposition,

hem-0.28 parts of FezOs in the-original product :to 0.0078 in the reds-posited salt; 0.9 parts of -SiO-2 in the original product to 0.04 in the redeposi-ted salt. The yield of alumina exceeded per cent.

What we claim as-our invention and desire to secure by Letters Patent. is: l

:1. A process for theproduction of monobasic aluminum sulphite which comprises. .j heating aluminum sulphite solutions: under conditions wholly maintaining the S02 content and iii-which at least two molecules -of S02 are present to one molecule of until the monobasic aluminum sullphi't'e is deposited from the solution. 1

-2. A process for the production of lnonobasic aluminum s-ul'phite which comprises heating aluminum ssulphite solutions under conditions partially maintaining the S02 content and in which at least two molecules of 302 are present to one molecule .of A1203, until the monoha-sic alumin um sulphite is deposited from the solution. r

3. A process forthe production of inonobasic aluminum sulph'ite which comprises heating aluminum sulphite solutions under conditions wholly maintaining the S02 content. and. in which at least two molecules of S02 are present to one molecule of A1203, until the :monob'asic aluminum sulphite is deposited fromthe solution-s, red-issolving said basic aluminum sulphite which still contains iron, in aqueoussulphurous acid and redepositing it from "the solutionwby heating under conditions wholly maintaining the E02 content.

l. A process for the production of monebasic aluminum sulphite which comprises heating aluminum sulphite solutions under conditions wholly maintaining the S02-contentand in which at least two molecules of S02 are present to one molecule of A1203, until the 'monoba'sic aluminum sulphite is deposited from the solutions, redissolving said basic aluminum sulphite which still contains iron, in aqueous sulphurous acid and redepositing it from the solution by heating under conditions partially maintaining the S02 content, at least two molecules of S02 being present for each molecule of A1203 during said redeposition.

5. A process for the production of monobasic aluminum sulphite which comprises heating aluminum sulphite solutions under conditions partially maintaining the S02 content and in which at least two molecules of S02 are present to one molecule of A1203, until the monobasic aluminum sulphite is deposited from the solutions, redissolving said basic aluminum sulphite which still contains iron, in aqueous sulphurous acid and redepositing it from the solution by heating under conditions wholly maintaining the S02 content.

6.'A process for the production of monobasic aluminum sulphite which comprises heating aluminum sulphite solutions under conditions partially maintaining the S02 content and in which at least two molecules of S02 are present to one molecule of A1203, until the monobasic aluminum sulphite is deposited from the solutions, redissolving said basic aluminum sulphite which still contain iron, in aqueous sulphurous acid and redepositing it from the solution by heating under conditions partially maintaining the S02 content, at least two molecules of S02 being present for each molecule of A1202 during" said redeposition.

'7. A process for the production of monobasic aluminum sulphite which comprises depositing "JOY basic aluminum sulphite mixtures low in S02 from aluminum sulphite solutions by hydrolytic separation, redissolving the deposition in concentrated or dilute aqueous sulphurous acid and heating the solution under conditions wholly maintaining the S02 content in which solution at least two molecules of S02 are present to one moleoule of A1203.

8. A process for the production of monoloasic aluminum sulphite which comprises depositing basic aluminum sulphite mixtures low in S02 from aluminum sulphite solutions by hydrolytic separation, redissolving the deposition in concentrated or dilute aqueous sulphurous acid and heating the solution under conditions partially maintaining the S02 content in which solution at least two molecules of S02 are present to one molecule of A1203.

9. A process for the production of pure monobasic aluminum sulphite of definite granular size, which comprises heating aluminum sulphite solutions at temperatures ranging between about 70 and 150 C. under conditions Wholly maintaining the S02 content and in which at least two molecules of S02 are present to one molecule of A1203 until the monobasic aluminum sulphite is deposited from the solutions.

10. A process for the production of pure monobasic aluminum sulphite of definite granular size, which comprises heating aluminum sulphite solutions at temperatures ranging between about 70 and 150 C. under conditions partially maintaining the S02 content and in which at least two molecules of S02 are present to one molecule of A1202 until the monobasic aluminum sulphite is deposited from the solutions.

11. In the process of making monobasic aluminum sulphite, the step which comprises heating a solution of aluminum sulphite containing at least two molecules of S02 for each molecule of A1203, under atmospheric pressure, to temperatures below 90 C. and under conditions maintaining a molecular ratio of at least 2 to 1 between S02 and A1202, thereby precipitating said monobasic aluminum sulphite.

12. The process of claim 1 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

13. The process of claim 2 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

14. The process of claim 3 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

15. The process of claim 4 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

16. The process of claim 5 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

17. The process of claim 6 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

18. The process of claim '7 wherein the basic aluminum sulphite is deposited from impure solutions containing silica and iron as impurities.

19. The process of claim 8 wherein the basic aluminum sulphite is deposited from impure solutions containing silica and iron as impurities.

20. The process of claim 9 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

21. The process of claim 10 wherein the aluminum sulphite solutions heated are impure products containing silica and iron as impurities.

ERICH WIEDBRAUCK.

KARL BUCHE. 

